1. Field of the Invention
The invention relates generally to a dual damascene process for forming a contact hole for an integrated circuit (IC) and more particularly to a dual damascene process that uses a sacrificial layer of spin-on material for protecting the contact hole profile during the damascene etch process.
2. Description of the Related Art
A conventional dual damascene process is described in Licata et al., "Dual Damascene Al Wiring for 256M DRAM," Proceedings of the 12th International VLSI Multilevel Interconection Conference, edited by T.E. Wade (VMIC, Tampa), pp. 596-602 (1995). FIGS. 1A-1F illustrate the steps of the conventional dual damascene process.
FIG. 1A illustrates an oxide layer 20 disposed on top of a semiconductor substrate 10. A layer of photoresist 30 is spin coated on top of the oxide layer 20, exposed through a mask (not shown) containing a contact hole pattern, and developed. An etch opening 31 is thus formed. Using the remaining photoresist as an etch mask, the oxide layer 20 is then etched to form a contact hole 32 and the remaining photoresist is then removed producing the structure shown in FIG. 1B.
Next, a layer of photoresist 33 is applied a second time (FIG. 1C). The photoresist 33 is then exposed through a mask (not shown) and developed to form an etch opening 34 (FIG. 1D). Using the remaining photoresist as an etch mask, the oxide layer 20 is etched to form a wiring trough 35 as shown in FIG. 1E. This etch step is known as a damascene etch step. The remaining photoresist is removed and the contact hole 32 and the wiring trough 35 are filled with metal 36.
The above-described process is difficult to control for three reasons.
First, the reflectivity from the topography substrate makes the width of the etch opening 34 in the photoresist 33 difficult to control.
Second, as feature sizes have become smaller, the aspect ratio (height/width) of the contact hole 32 has increased. At high aspect ratios, it is difficult for the photoresist 33 to completely flow into and fill the contact hole 32. If the contact hole is not completely filled, there is a possibility that the photoresist 33 disposed within the contact hole 32 may partially or even completely develop away and provide little or no protection for the contact hole profile during the subsequent damascene etch step.
Third, the thickness of the photoresist 33 over the topography substrate varies signficantly, and so the exposure depth of the photoresist 33 is difficult to control. This is likely to cause an over-development of the photoresist 33 disposed within the contact hole 32 and possibly erode the contact hole profile during the subsequent damascene etch step.